There is a joining method in which when works such as aluminum alloy plates are joined, a rod-shaped tool (a tool having a shoulder portion having a large diameter and a probe projectingly provided at the tip thereof) rotating at a high speed is pressed against and inserted into one end of a butting portion formed by butting joint surfaces of the works, the tool, while rotating at a high speed, is moved to the other end along the butting portion, and the butting portion is softened by frictional heat generated at this time, thereby joining the works. This method is a technique called friction stir welding (FSW).
According to the friction stir welding, the maximum ultimate temperature does not reach a melting point and joining is performed in a solid phase state because joining is performed utilizing frictional heat between a tool and a work. Therefore, there is the advantage that as compared to melt-welding such as arc welding, a decrease in strength of a joint is small, joining defects such as voids and cracks do not occur, the joint surface is flat, and so on.
Further, there is a modification method in which a probe of a tool rotating at a high speed as described above is strongly pressed against and inserted into the surface of a work such as an aluminum alloy plate, the tool, while rotating at a high speed, is moved, and the work in the vicinity of a shoulder portion and the probe of the tool is softened by frictional heat generated at this time, thereby decreasing the crystal grain size of the work up to a certain depth to improve strength, hardness and the like. This method is called friction stir processing (FSP). Further, there is a spot joining method in which a tool is pressed against a work but is not laterally moved, and is pulled out as it is after a certain time. This method is called friction spot joining (FSJ). A process in which a rotating tool is strongly pressed against a work, and the work is processed by frictional heat thus generated as in the above-mentioned FSW, FSP, FSJ or the like is referred to as friction stir processing.
A tool made of steel such as SKD steel is used for a tool when aluminum or an aluminum alloy is used as a work in the friction stir processing. However, the tool made of steel such as SKD steel has the problem that it is soon deformed due to attrition or the like, so that joining cannot be performed. A tool made of ceramic has the problem that it is expensive and is easily broken, and it is easily worn away particularly when the work is stainless steel. If very small pieces of a material of a tool made of ceramic are dispersed in an iron-based work, for example stainless steel when the tool is worn away due to friction stir processing, mechanical properties and corrosion resistance may be degraded.
On the other hand, as a friction stir processing tool, one made of a Ni-based dual multi-phase intermetallic compound alloy has been proposed (for example Patent Document 1). The Ni-based dual multi-phase intermetallic compound alloy which is a material of this tool includes a Ni3Al—Ni3Nb—Ni3V-based intermetallic compound alloy or a Ni3Al—Ni3Ti—Ni3V-based intermetallic compound alloy. The Ni-based dual multi-phase intermetallic compound alloy is a multi-phase alloy formed by combining Ni3X type intermetallic compounds, and has excellent hardness as compared to an alloy formed of a single intermetallic compound phase. Therefore, a friction stir processing tool made of a Ni-based dual multi-phase intermetallic compound alloy, is particularly suitable for friction stir processing of a work of iron, an iron alloy or the like which requires a high processing temperature because necessary hardness is maintained even when the temperature is increased (to 800° C. or higher) by frictional heat during processing to the extent that the tool side face emits light in orange color.